1. Reservation of Copyright
The disclosure of this patent document contains material, which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the U.S. Patent and Trademark Office patent files or records, but otherwise reserves all copyright rights whatsoever.
2. Related Application Data
Priority is hereby claimed to U.S. Provisional Patent Application No. 60/177,658, entitled SYSTEM AND METHOD FOR PROVIDING POLARIZATION MATCHING ON A CELLULAR COMMUNICATION FORWARD LINK, filed on Jan. 27, 2000, the contents of which is hereby expressly incorporated herein by reference thereto, in its entirety.
3. Field of the Invention
This invention generally relates to the field of cellular communications. More particularly, the present invention relates to a system and method that provides polarization matching on a forward link transmission of a cellular communication system.
4. Description of Background Information
Today's cellular communication systems are subject to ever-increasing user demands. Current subscribers are demanding more services and better quality, while system capacities are being pushed to their limits. In striving to achieve higher capacities and better grades of service, it becomes necessary to optimize transmission integrity by decreasing transmission losses wherever possible.
Typically, for each geographic cell, cellular communication systems employ a base station (BS) with an omni-directional antenna that provides signal coverage throughout the cell. Alternative approaches include angularly dividing the geographic cells into sectors (i.e., sectorization) and deploying BS antennae that radiate highly-directive narrow beam patterns to cover designated sectors. The directive beam patterns can be narrow in both the azimuthal and elevation plane and, by virtue of their directional gain, enable mobile stations (MSs) to communicate with the BS at longer distances.
Such narrow beams are used to form beam patterns for given coverage areas or geographic cells and are optimized to improve performance of the wireless network. Optimization may include the polarization of the beams to enhance performance. It will be appreciated that polarization of a radiated pattern in a specified direction results in the maximum gain of the pattern along the specified direction. Traditionally, BSs employ a plurality of vertically-polarized antennae, which complement the vertically-polarized antennae of most mounted MSs (e.g., non-handheld MSs). This configuration, however, is insufficient in accommodating signals from handheld MSs.
Generally, handheld MSs contain transmit antennae that are linearly polarized. The linear polarization may be randomly distributed, depending on the position in which the handheld MS is physically held. For example, depending on the physical orientation of the handheld MS during use, the polarization of the handheld MS antenna may vary up to 20° degrees from the horizontal to the vertical plane relative to the BS antennae. Such variance translates into an a priori polarization mismatch between handheld MSs and BS antennae. Such a polarization mismatch can cause average transmission losses of up to 7 dB and instantaneous transmission losses of up to 9 dB.
In an effort to overcome such costly transmission losses, some BSs incorporate multiple polarization-diverse antennae with combiner circuitry to attempt to “match” the polarization of the MS-to-BS (i.e., reverse link) transmissions. For example, FIG. 1 illustrates a receive portion of a BS antenna system 100 employing polarization-diverse receive antenna elements 102, 104. The receive elements 102, 104 are configured to accommodate two opposing linearly-slanted polarized states (i.e., ±45° linear polarization). The signals received by both types of antenna elements 102, 104 are applied to a diversity combining circuit 106, which determines a maximum value in a preferred signal parameter (e.g., signal-to-noise ratio, SNR) between the two signals received from antenna elements 102, 104. In this manner, the BS is able to limit transmission losses due to polarization mismatches by matching the reverse link transmission to one of two possible polarization states (i.e., ±45°).
Because handheld MSs are not generally equipped with multiple antennae, such polarization matching schemes cannot be implemented in MSs to compensate for polarization mismatches during (i.e., forward link) BS-to-MS transmissions. This results in costly transmission losses in the forward link. What is needed, therefore, is a system and method that provides effective polarization matching on forward link transmissions to mitigate transmission losses due to the a priori polarization mismatch between handheld MSs and BSs.